A conveyor roll used in high temperature applications may comprise a ceramic spool having metal end caps. Typically, the ceramic spool comprises fused silica. The end caps permit facile mounting to a bearing or drive wheel. For example, in glass tempering applications, the ceramic spool supports glass sheet, and the end caps permit mechanical coupling to the drive mechanism. The end caps should securely adhere to the spool, thereby permitting the spool to rotate at the desired speed. Eccentric rotation is generally undesirable, as this would create an uneven support surface for the glass sheet.
The different thermal expansions of the ceramic spool and the metal end caps makes securely fastening the end caps to the spool difficult and can create eccentric rotations. Various methods have been proposed to overcome this difficulty. U.S. Pat. No. 3,867,748 teaches fastening end caps to a spool using an adhesive. U.S. Pat. No. 4,242,782 proposes fastening end caps using rubber O-rings. Adhesives and O-rings can become pliable and lose holding power at elevated temperatures causing eccentric rotation of the spool and slippage between the end caps and the spool. If, by accident, the adhesive and O-ring have been subjected to such elevated temperatures, they lose definitely their holding power so that even when the temperature returns to its normal value, slippage is still observed. For this reason, the fastening of end caps to a spool using adhesive or O-ring is limited to applications at low temperature (lower than 250° C.).
Metallic connectors have also been used to secure an end cap to a spool. U.S. Pat. No. 5,316,129 or U.S. Pat. No. 4,404,011 describes the use of a helically wound coil between the spool and the end cap. The coil includes bent portions and straight portions that permit continuous contact between the end cap and the spool despite disparate thermal expansion coefficients. The flat sided spring coil of this document consists in a succession of relatively long portions which are relatively thin. In these conditions, a significant flexibility of the coil elements is observed. Consequently, to obtain a torque sufficient to secure the end cap to the spool, it is necessary to pre-stress the coil very significantly. If the temperature increases accidentally (temporary overheat), the torque and consequently the fastening of the end cap to the spool are lost. Moreover, when the system cools down, the torque is not recovered. U.S. Pat. Nos. 5,906,567 and 5,370,596 describe curved bimetallic shims for securing the end cap to the spool. The curvature of the bimetallic shims changes with temperature thereby retaining a secure attachment between the spool and the end cap. Assembling and repairing a coil spring or bimetallic system can, however, be difficult. In addition, such metallic connectors are particularly adapted to specific temperature ranges (generally 400° C. and higher). Outside of this range, eccentric rotation can be observed.
Mechanical fasteners can be used to fasten an end cap to a spool. U.S. Pat. No. 4,751,776 shows an end cap having an annular distribution of screws that thread into a ferrule on the end of a spool. The screws are tightened to secure the end cap to the spool, but thermal expansion and contraction can loosen the screws causing eccentric rotation and slippage. In this case, the screws will seriously damage the surface of the spool. Screws have also been keyed to a flat surface machined on face of the spool so as to ensure coaxial rotation. Assembling and repairing such sleeve and locking key is time consuming. FR 2 550 172 describes an end cap including a metallic collar between the spool and the end cap. The end cap is locked in place with a key fitted to a flat surface of the spool. The collar has a thermal expansion greater than the end cap, and is intended to compensate for the difference in thermal expansion between the end cap and the spool. U.S. Pat. No. 5,146,675 discloses a screw that can be tightened to force a metal plate against a flat on the spool, which presses the spool against the inner surface of the end cap. The end cap includes an access opening that permits the screw, metal plate and end cap to be welded together, thereby preventing the screw from backing out. Effectively, the end cap and the spool connect across a single axis, that is, the screw. Thermal cycling across a single axis of contact can cause eccentric rotation and movement of the metal plate relative to the spool. Such movement can even cause the plate to contact the edge of the flat and crack the spool.
Another problem which is often observed with the conveyor rolls of the prior art is that of jamming. For example, in case the conveyed article is blocked or in case of mechanical seizing, the drive mechanism will continue to operate and will transmit a moment of torsion to the end cap. In these conditions, either the ceramic spool or the fastening means will break. The same consequences are observed on the long run when the conveyor roll is subjected to a brutal acceleration or deceleration.
A need persists for an end cap that fixedly and centrally secures to a ceramic spool within a wide range of application temperatures. The end cap should be able to resist a temporary overheat and to recover its holding power when the temperature is back to normal without causing eccentric rotation. The end cap should also be easy to install. The conveyor roll should also be able to resist temporary jamming or seizing of the line as well as to brutal acceleration or deceleration.